The present invention relates to a process for the preparation of six-membered ring carbocycles by ring hydrogenation of aromatic compounds using hydrogen, the process being carried out in a reaction system consisting of two liquid, immiscible phases, of which the first phase is formed by the aromatic compound and, if necessary, a water-immiscible solvent, and the second phase consists of water, and, colloidally dispersed therein, a hydrogenation-active metal as hydrogenation catalyst, and auxiliaries for stabilizing the colloidal catalyst. Six-membered ring carbocycles of the type which can be prepared according to the invention are intermediates, known to the person skilled in the art, for the preparation of active ingredients and industrial chemicals.
It is already known to prepare six-membered ring carbocycles by ring hydrogenation of aromatic compounds. The hydrogenation is generally carried out using catalytically activated hydrogen; the hydrogenation-active metals suitable for this purpose are known and are generally those from the group of platinum metals and also nickel, chromium, niobium or copper. Although catalytic hydrogenations in the homogeneous phase are characterized by low diffusion inhibition, they are also characterized by the difficulty in separating the reaction product off from the reaction mixture and thus from the homogeneously dissolved catalyst by distillation, crystallization or other methods. The activity of the hydrogenation catalyst frequently also suffers at the same time. Use of slurried catalyst in the slurry phase partly overcomes the difficulty of separation although the need for coarse and fine filtration remains. There does however remain the risk of deactivating the catalyst, and a new difficulty arises, namely that of erosion in pumps, piping, valves and other parts of the apparatus by the catalyst to be removed. Difficulties of the last-mentioned type are overcome by arranging the catalyst in the form of a fixed bed in the reaction apparatus and allowing the reaction material in the liquid phase (e.g. trickle phase) or in the gas phase to flow over the catalyst. However, this reaction method, which has been perfected to a high degree, is only suitable for reactions with large throughputs. In the case of products which are prepared in small amounts and often only in batches, to carry out the reaction using catalysts in the form of a fixed bed is too complex.
DE-A 44 43 705 discloses the hydrogenation of benzene on a sulfobetaine-stabilized, colloidal catalyst comprising Ru on a La.sub.2 O.sub.3 support to give cyclohexane; at 50 bar and 150.degree. C., only 8.5% conversion and 78.5% selectivity were achieved in 0.5 h. According to Tetrahedron Letters 1983, 4139-42, the hydrogenation results deteriorate severely in the case of substituted benzenes. WO 96/08462 discloses the catalytic hydrogenation of aromatic amines to cyclohexylamines using hydrogen on an Ru/Al.sub.2 O.sub.3 catalyst in the presence of LiOH, which is said to prevent deamination; the reaction medium is a water-containing water-miscible organic solvent.